characteristic number

A characteristic number is a unique identifier assigned to a specific requirement, feature, or attribute on an engineering drawing or specification. In regulated manufacturing environments, it is commonly used to link each requirement to inspection records, data entry fields, or quality documentation for traceability.

How characteristic numbers are used

In practice, characteristic numbers typically:

• Appear on ballooned or numbered engineering drawings next to dimensions, notes, tolerances, and other requirements
• Map directly to line items in inspection forms, electronic inspection plans, or FAI (First Article Inspection) reports
• Provide a stable reference for recording measured values, results (pass/fail), and any associated nonconformances
• Support traceability between design requirements, shop floor inspection activities, and quality records in MES, QMS, or FAI software

Each characteristic number should be unique within the scope of the drawing or inspection plan so that a reviewer can unambiguously connect the documented result back to the original requirement.

What a characteristic number is not

• It is not the measured value itself; it is a reference that points to the requirement being measured.
• It is not inherently a risk or criticality rating, although systems may separately flag some characteristic numbers as key, critical, or safety-related.
• It is not limited to dimensional data; it can also identify notes, material requirements, finishes, processes, or documentation checks.

Common context in aerospace and AS9102

In aerospace first article inspection (AS9102), each requirement on the ballooned drawing is assigned a characteristic number. That number is then used in the AS9102 Form 3 (characteristic accountability and verification details) to:

• Identify the requirement being verified (dimension, note, specification, etc.)
• Record measured results and inspection methods
• Document any nonconformances related to that requirement

This linkage helps auditors and customers confirm that every identified requirement on the drawing has a corresponding inspection record, without implying approval or compliance on its own.

Common confusion

• Characteristic number vs. balloon number: On many drawings these are effectively the same, because each balloon on the drawing contains the characteristic number. Some organizations use the terms interchangeably.
• Characteristic number vs. characteristic ID in software: Digital systems may generate internal IDs that differ from the visible drawing number. In that case, the system should still preserve the original characteristic number as a reference back to the drawing.
• Characteristic number vs. critical characteristic: A critical or key characteristic is a classification of importance or risk. The characteristic number is simply the identifier; separate attributes or flags usually indicate criticality.